Micron-size metal-binding hydrogel particles improve germination and radicle elongation of Australian metallophyte grasses in mine waste rock and tailings

Abstract

Metal contamination of landscapes as a result of mining and other industrial activities is a pervasive problem worldwide. Metal contaminated soils often lack effective vegetation cover and are prone to contaminant leaching and dispersion through erosion, leading to contamination of the general environment. Hydrogels and similar hydrophilic and negatively-charged compounds are well-established ameliorants for wastes and tailings, but their application is constrained by transport (bulk), attributes, and cost. In this paper we demonstrate that metal-binding hydrogel particle amendments could be used to ameliorate substrates prior to planting, in order to enhance seedling emergence. In this study, micron-size thiol functional cross-linked acrylamide polymer hydrogel particles (X3) were synthesised and tested in laboratory-scale experiments on extremely saline and metal contaminated mine waste rock and tailings to determine: (i) their capacity to increase substrate water holding capacity (WHC); (ii) their metal-binding efficiency and capacity to reduce metal availability to plants to below the phytotoxicity threshold; and (iii) their effect on the germination characteristics and early radicle development of two Australian metallophyte grasses, Astrebla lappacea and Austrostipa scabra, under limiting and non-limiting water conditions. Addition of X3 to waste rock (18.4% dry weight) and tailings (3.2% dry weight) increased the WHC of both substrates, by more than 536% and 174% respectively, over the tested pH range from 2.0 to 6.3. X3 also significantly (P<0.05) lowered soluble concentrations of Al and Cu in leachate from amended waste rock, by up to 55 and 59% respectively between pH 3.2 and 6.3. Below pH 3.2, metal-binding efficiency declined to almost zero, suggesting a loss of particle functionality in extremely acidic conditions. X3 was not toxic to seed germination and significantly (P<0.05) increased germination percentages of A. lappacea and A. scabra in both waste rock and mine tailings in Petri dishes under controlled conditions. The highest germination percentages were recorded under a restricted water regime (substrates were watered to field capacity on the first day of the experiment but received no additional water thereafter). In A. lappacea, germination percentages increased to 43 and 10% in amended waste rock and tailings respectively compared to 9 and 0% in unamended treatments. In A. scabra, germination percentages increased from 0.5 to 24%, and from 6 to 21% in waste rock and tailings respectively. X3 also significantly (P<0.05) enhanced the radicle elongation of both species in contaminated waste rock and tailings. Under restricted water regime, radicle length of A. lappacea was increased to up to 19 and 13.5 mm in amended waste rock and tailings respectively as compared with 2 and 3 mm in unamended treatments. In A. scabra, radicle length was increased to up to 5.5 and 4.5 mm in amended waste rock and tailings respectively versus 0 and 1 mm in unamended treatments. https://papers.acg.uwa.edu.au/p/1...